1. Field of the Invention
This invention relates to a high definition television system, more particularly to a noise reduction circuit for reducing a noise in a high frequency range overlapping an output signal of a high definition television camera. The invention also relates to a high definition television image displaying system in which divided image signals in accordance with a high definition television system are displayed simultaneously on a screen composed of a plurality of present television receiver sets to form one. According to one aspect of the present invention, an image signal from another NTSC television equipment can be selectively supplied to one of the plurality of the present television receiver sets for display.
2. Description of the Prior Art
As a new television system standard for replacing the present television system standard, there has been proposed a high definition television system, for example, a Hi-vision system capable of providing a high degree of visual and psychological effects such as a sense of immediacy and a sense of power, which are superior to that of the present television system as well as being improved in picture quality and sound quality.
Because the Hi-vision system differs considerably from the present television system, such as an NTSC television system, in its screen displaying system, scanning system, sound reproducing system and the like, it is impossible to reproduce an image of a Hi-vision signal simply by using an NTSC television receiver. Therefore, there have been proposed various displaying systems, wherein differences in a number of scanning lines, aspect ratio, horizontal scanning frequency and the like between these two systems are assimilated to that of the NTSC television signal for displaying the Hi-vision signal.
FIGS. 3A and 3B are illustrations of screen configurations for illustrating an example of a Hi-vision/NTSC television signal conversion and display system as described above, wherein a Hi-vision screen (FIG. 3A) having 1,125 horizontal scanning lines and an aspect ratio of 16:9 is divided into the total of twelve screens, three vertical screens by four horizontal screens, by using a scan converter. Hi-vision signals being divided for each of the screens are displayed on one composite screen (FIG. 3B) comprised of the twelve NTSC television receiver sets #1 to #12 after converting them into NTSC television signals, whereby each NTSC television screen has 525 horizontal scanning lines and an aspect ratio of 4:3. If multi-display projection units are used for the NTSC television receiver sets #1 to #12, a high definition television image can be displayed on the composite screen with sufficient brightness for practical use.
However, in order to convert the Hi-vision signal into the NTSC television signals for display, it is necessary to decrease a number of scanning lines of the Hi-vision signal, therefore it is impossible to reproduce an image of high picture quality. In view of this fact, there has been proposed a displaying system in which one Hi-vision image is divided into a plurality of images, the divided images are then displayed on one composite screen composed of a plurality of NTSC television receiver sets, thereby an image of high picture quality is reproduced without reducing a number of scanning lines (See Ja. Pat. Appln. No. Sho 63-92987). In addition, there has been also proposed another displaying system for displaying an image from other NTSC television equipment on one or more of a plurality of NTSC television receiver sets (See Ja. Pat. Appln. No. Hei 1-43173).
In the displaying system shown in FIG. 3B, if a Hi-vision signal having a band width of 20 MHz is spreaded on twelve screens through a scan converter, the Hi-vision signal can be converted into a plurality of NTSC television signals, each of which has a band width of 2.33 MHz. In this case, if an output signal of a Hi-vision camera is converted directly into the NTSC television signals for twelve screens, there arises a problem such that a noise in the high frequency range, approximately 20 MHz, which has not been noticeable on the Hi-vision screen, becomes noticeable on the converted NTSC television screens due to the fact that the output signal of the Hi-vision camera includes a noise (a triangular noise), the an amplitude of which increases in proportion to a frequency increase.
This problem is caused by the fact that the spreading of the Hi-vision signal having the band width of 20 MHz on twelve screens through a scan converter converts the Hi-vision signal into the NTSC television signals having the band width of 2.33 MHz and that a noise of 20 MHz simultaneously into about 2 MHz at which frequency the visual sensitivity of human eyes is found to be the highest. Thus, the noise in the high frequency range, which has not been noticeable on the Hi-vision screen, becomes noticeable on the NTSC screens after the scan conversion.
Furthermore, when the Hi-vision image is divided into a plurality of NTSC television images and displayed on a plurality of NTSC television receiver sets, or an image from other NTSC television equipment is displayed on a plurality of NTSC television receiver sets, that is, in case that the Hi-vision image and the NTSC television image are displayed, by switching, on respective NTSC television receiver sets, it is necessary to supply respective synchronizing signals to the Hi-vision equipment for generating the Hi-vision signal and to the NTSC television equipment for generating the NTSC television signal in order to provide synchronization therebetween.
However, the Hi-vision signal generated by the Hi-vision equipment is delayed more than one field at a stage, into a plurality of NTSC television signals, thus resulting in a phase difference between the Hi-vision signal and the NTSC television signal which is generated by the NTSC television equipment. Therefore, if the Hi-vision image and the NTSC television image are changed over during display, there is caused an instantaneous picture disturbance at the time of changing over.
It is therefore an object of this invention to reduce noise in a high frequency range contained in a high definition television signal in advance of a scan conversion and to reduce an visually disturbing noise generated upon converting the high definition television signal into the present television signals.
It is another object of this invention to provide an image displaying system wherein a picture disturbance is prevented when performing a changing-over in displaying, after converting an image of the high definition television system into a plurality of images of the present television system, between the converted images and an image generated by another video equipment of the present television system.